A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube-to-flange joints subjected to combined loadings

Full penetration butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V-notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N-SIF) and assess the high cycle fatigue strength of the welded joints. The N-SIF-based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N-SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R-C and given in closed form as a function of the mode I and mode III N-SIFs. The value of R-C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R-C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re-analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine-grained Fe E 460 steel and of age-hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R-C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves.